1. Field of the Invention
Many types of intermetallic compounds are known for use as hydrogen storage materials. Of particular interest herein are hydrogen storage materials provided by three-component ZrMn.sub.2 -type alloys in which zirconium is partially substituted with cerium, praseodymium or neodymium, and which are characterized in having the C14 hexagonal crystal structure and AB.sub.2 stoichiometry.
2. State of the Art
A material suitable for storage of hydrogen must satisfy many demanding criteria. In addition to large storage capacity for hydrogen, a hydrogen storage material should absorb and desorb hydrogen quickly, preferably at a pressure near one atmosphere, and the material should show a minimum of hysteresis effects during a hydrogen absorption/desorption cycle.
Intermetallic compounds which have received much attention for use as hydrogen storage materials are provided by derivatives of ZrMn.sub.2 compounds, which compounds are characterized by hexagonal C14 crystal structure and AB.sub.2 stoichiometry. It is well known that the ZrMn.sub.2 system is capable of absorbing copious quantities of hydrogen, but that hydrides formed from the ZrMn.sub.2 system are too stable to be of practical significance. In search of improved ZrMn.sub.2 -type systems, alloys have been prepared which contain other elements substituted for all or a portion of the zirconium, but with the AB.sub.2 stoichiometry maintained in the new alloy. For example, in Shaltiel et al, J. Less-Comm. Metals, 53, 117-131 (1977), there are described changes in properties of AB.sub.2 Laves-phase ZrMn.sub.2 -based compounds by substitution of manganese with a 3d transition metal in accordance with the empirical formula Zr (Co.sub.x M.sub.1-x).sub.2 and Zr (Fe.sub.x M.sub.1-x).sub.2 wherein M=V, Cr, Mn and x is between zero and one. Other studies of partial substitution of zirconium in ZrMn.sub.2 alloys with titanium to form the hydrides of Ti.sub.1-x Zr.sub.x Mn.sub.2 pseudobinaries are described in Oesterreicher et al, Mat. Res. Bull, 13, 83-88 (1978). In Fujii et al, J. Phys. Chem., 85, 3112-16 (1981), ternary alloys are described of the type Zr.sub.1-x Ti.sub.x Mn.sub.2 wherein x=0 to 0.5.